Name_________________________________________________________
Chapter
10
Date __________
Probability and Statistics
Dear Family,
When you plan an outdoor event like a picnic, you cannot be certain that the
weather will cooperate. Almost immediately, you begin to wonder— will it be
warm or cool? sunny or cloudy? dry or rainy? There is no way to be certain,
so you turn to the weather forecast to find out what is likely. When weather
forecasters say there is a 60% chance of rain, do you ever wonder how they
know? The weather report introduces you to the concept of probability.
The National Weather Service keeps track of daily conditions. They record
the temperature, humidity, air pressure, and other data, including the weather
produced by those conditions. The forecasters compare this historical data
with current conditions and may see that out of 100 days with similar
conditions, 60 of them were rainy days.
In probability, a favorable outcome is the result you are looking for, such as
the number of rainy days. The ratio of the favorable outcome to the total
number of outcomes is the probability.
number of favorable outcomes
total number of outcomes

days with rain
total days

60
100
 0.6  60%
The next time you are relying on good weather, you may want to do your own
research. You and your student can think about these topics:

The Farmer’s Almanac provides historical weather information, such as
the number of times it rained on a given date. Use this information to
determine the probability that it will rain on the date of your event.

For some events, like a pool party or a picnic by the lake, you may want
to get a sense of what the temperature will be. What is the probability
that the temperature will be above 70 degrees the day of your event?

What other conditions and probabilities do you want to know?
You might revise your plans if the conditions aren’t favorable.
Pick a favorable day and then enjoy your picnic! Remember to watch for ants—
they are almost certain to attend!
Mrs. Authement
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